Shao Kahn is the evil emperor of Outworld. He has taken over many realms in the past, including the once beautiful realm known as Edenia. He conquered and destroyed it, forcing it to become part of the Dark Realm of Outworld. He murdered Edenia's king, took the Queen Sindel as his bride, adopted Kitana as his daughter and transformed Edenia into a desolate realm - making it part of his collection of realms knows as the Outworld.
He seeks out other worlds to conquer and he desperately wants the Earth realm, not just as a collector, but because Earth is special, a young energy rich planet that also happens to be the gateway to other realms.